WO2002031903A1

WO2002031903A1 - Porous carbon body for a fuel cell and method of manufacture

Info

Publication number: WO2002031903A1
Application number: PCT/US2001/030357
Authority: WO
Inventors: Wayde R. Schmidt; Albert T. Pucino; Bardia Guilani; Myron Krasij; Ned E. Cipollini; Brian F. Dufner
Original assignee: International Fuel Cells, Llc
Priority date: 2000-10-06
Filing date: 2001-09-26
Publication date: 2002-04-18
Also published as: AU2001293157A1

Abstract

The invention is a porous carbon body (12, 14) and method of manufacture of the body for usage in a fuel cell. The porous carbon body (12, 14) comprises an electrically conductive graphite powder in an amount of between 40%-60% by weight of the body; a carbon fiber in an amount of between 20%-40% by weight of the body; a hydrophobic binder in an amount of between 10%-30% by weight of the body; wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25%; and, wherein the pores in the body are rendered partially hydrophilic by incorporation of a hydrophilic rendering compound onto an interior surface of the pores. the compound is a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate. The porous carbon body (12, 14) achieves long term chemical stability for operation in a PEM fuel cell.

Description

POROUS CARBONBODYFORAFUEL CELL ANDMETHOD OFMANUFACTURE

Technical Field

The present invention relates to fuel cells that are suited for usage in 5 transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a porous carbon body that may be used within a fuel cell for transporting reactant, product and coolant fluids to, through and from the fuel cell, for conducting electricity from one cell to an adjacent cell, for providing a fluid barrier to gaseous reactants, for defining gaseous reactant distribution channels, o and/or for providing mechanical integrity to the fuel cell.

Background of the Invention

Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactant fluids to power electrical apparatus such as apparatus on-board space vehicles, or on-site generators for buildings. A plurality of 5 planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids as part of a fuel cell power plant. Each individual fuel cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, 0 and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane ("PEM") as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through 5 the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.

The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton o exchange membrane ("PEM") electrolyte, which consists of a solid polymer well- known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the 5 PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.

In operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode ("product water") including water resulting from proton drag ("drag water") through 0 the PEM electrolyte and rates at which water is removed from the cathode and at which water is supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is 5 returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and o hence decreasing current flow. Additionally, if too much water is removed from the cathode, the PEM may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance. Another difficulty associated with fuel cells is related to usage of a coolant system necessary to maintain an operating fuel cell within an appropriate heat range. Heat must be removed from the fuel cell, and it is 5 common to pass a cooling fluid through cooler plates adjacent to reactant stream flow fields of the fuel cell to remove heat from the fuel cell.

It is known to utilize one component of a fuel cell to assist in the accomplishment of a variety of the aforesaid water management and related tasks. Such a component is typically formed of a porous carbon body and is commonly o referred to under various names including "cooler plate", "water transport plate",

"separator plate", "bi-polar plate", "end plate", among other names. For example, in U.S. Patent 6,024,848 that issued on Feb. 15, 2000 to Dufher et al., a water transport plate is shown that defines a plurality of coolant water feed channels on a planar surface of the plate and on an opposed surface a network of reactant gas distribution 5 channels is defined. Such a water transport plate is a typical porous carbon body and the plate must perform a variety of functions. It must transport water from coolant channels through the body to gaseous reactant channels to humidify a reactant fluid within the gas reactant channels; it must remove product water generated at the cathode electrode across the body into the coolant water channels to prevent flooding 5 of the cathode electrode; it must form a gaseous barrier to prevent mixing of fuel and oxidant reactant fluids on opposed sides of the plate; it must conduct electricity or electrons from one cell to an adjacent fuel cell in a fuel cell stack assembly; it must conduct waste heat generated at the cathode away from the cathode to the coolant fluid; it may provide a distribution network for oxidant and reducing fluid reactants; 0 and, it must provide mechanical support and integrity to the fuel cell.

Therefore, a porous carbon body that makes up such a water transport plate must be porous, wettable to water, have a high rate of water permeability, have a high bubble pressure, be a good electrical and thermal conductor, have good compressive and flexural strength, and the porous carbon body must be chemically stable in the s environment of an operating PEM fuel cell. Some of these qualities require characteristics that are inconsistent with characteristics appropriate for other such qualities. For example, to increase bubble pressure to thereby enhance a gaseous seal between gaseous oxidant and fuel reactants on opposed sides of the porous carbon body, it is appropriate to have a small mean pore size of the pores within the body. o However, to enhance permeability of the body to coolant or product water, it is desirable to have a large mean pore size. Similarly, a high porosity, or per cent open pore volume, is appropriate for enhancing flow of water through the porous carbon body, however a high porosity is detrimental to both electrical conductivity and mechanical strength. Where the porous carbon body includes a resin, it is desirable to 5 have a resin that is wettable to water to enhance performance of the body during operation of the fuel cell, however such a wettable resin is undesirable from a material compatibility perspective, because wettable resins dissolve more quickly in water hence limiting a useful life of the body.

Many approaches are known to produce an efficient porous carbon body for o use in a PEM fuel cell that takes into consideration many of these requirements and limitations. For example, U.S. Patent 5,840,414 that issued on November 24, 1998 to Bert et al., shows a porous carbon body that achieves increased wettability by incorporation of a metal oxide into the pores of the body, wherein the metal oxide has a solubility in water of less than about 10^"6 moles per liter. While Bert et al. improves 5 one aspect of a porous carbon body for a PEM fuel cell, as with most known porous carbon bodies, the carbon plate that is utilized in Bert et al. is graphitized. Normal manufacture of graphitized carbon bodies includes a very high temperature, lengthy process in order to produce a crystallized graphite structure. For example, to graphitize a mixture of a graphite powder and a resin into a porous carbon body 5 acceptable for use in a PEM fuel cell, it is known to first compress the mixture in a mold to establish a pre-determined porosity at about 100 - 500 (690 Kpa - 3447 Kpa) pounds per square inch ("p. si.") and at about 325 - 375 degrees Fahrenheit ("°F") (163°C - 190°C), and to then heat the molded body in an inert atmosphere at about 3,600 - 5,400 °F (1982°C - 2982°C). As can be easily understood, such a process is 0 quite expensive and time consuming often taking several weeks, and hence is a substantial problem in providing a cost effective porous carbon body for a PEM fuel cell.

Austrian Patent 389,020 issued on February 15, 1989 to Schutz also discloses a porous carbon structure that may be utilized in a variety of roles within a fuel cell, 5 including usage as a transport medium for water, and usage as an end plate secured between a cathode of a first cell and an anode of a second, adjacent cell. The porous carbon body is prepared by mixing with finely ground graphite or carbon black a pore formation agent such as baking powder or sugar, and or 0 - 50% microporous fillers such as kaolin or asbestos. The mixture is then mixed with 0 - 10% sodium o carboxymethycellulose and suspended in a polysulfone solution. That mixture is then applied to a carrier substance, then dried and sintered, and the pore formation agents are removed by boiling and/or washing. The resultant porous carbon body is described as having a majority of pores having a size of 0.001 - 1 microns. While the Schutz porous carbon structure describes some valuable characteristics for application 5 in a fuel cell, the experience of the present inventors is that such porous carbon bodies operate satisfactorily for a limited duration, but eventually the bodies becomes non- wetting, or hydrophobic, and hence unable to prevent cross-over through the body of gaseous reactant fluids resulting in a dangerous mixing of the reactant fluids.

Another approach to producing a porous carbon body for a PEM fuel cell is o disclosed in U.S. Patent 5,942,347 that issued on August 24, 1999 to Koncar et al. wherein the body is described as a "bi-polar separator plate". The plate includes 50% to about 95% by weight of a preferably carbonaceous "electronically conductive material", at least 5% by weight of a resin, and at least one hydrophilic agent wherein the conductive material, resin and hydrophilic agent are "substantially uniformly 5 dispersed throughout" the separator plate. In formation of the Koncar et al. plate, the hydrophilic or wetting agent is mixed together with the electronically conductive material and resin to produce a "uniform dispersion" of the wetting agent, and the mixture is then molded into a plate at 500 - 4,000 p.s.i. (3447 Kpa - 27,580 Kpa) and 250 - 800°F (121°C - 427°C) so that the wetting agent "promotes the formation of 5 pores in the molded product by preventing the resin and other components from forming one continuous phase" (Col. 6, lines 34 - 38). Koncar et al. discloses specific water absorption or hydrophilicity characteristics as a per cent weight gain based upon usage of an oxide of silica as the wetting agent, and Koncar further describes a "substantial water absorption" as an 18% weight gain. While the Koncar et al. bi- 0 polar separator plate may achieve some increased wettability and adequate porosity and mean pore size, it is suspected that the plate cannot provide effective water transport and provide the other necessary functions for a porous carbon body within a PEM cell operating up to 1,000 - 2,000 amps per square foot ("ASF") 1.1 - 2.15 amps per square cm, and it is further doubted that the Koncar et al. plate can provide s adequate long-term chemical stability for operations exceeding 1,500 hours at temperatures of about 180°F (82.2°C).

Accordingly, there is a need for a porous carbon body for a fuel cell that may be efficiently manufactured, and that provides appropriate porosity and mean pore size to support effective water transport, thermal and electrical conduction, and o mechanical strength necessary for operation of a PEM fuel cell.

Disclosure of the Invention

The invention is a porous carbon body and method of manufacture of the body for usage in a fuel cell. The porous carbon body comprises an electrically conductive graphite powder in an amount of between 40% - 60% by weight of the body; a carbon 5 fiber in an amount of between 20% - 40% by weight of the body; a hydrophobic binder in an amount of between 10% - 30% by weight of the body; wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25%; and, wherein the pores in the body are rendered partially hydrophilic by incorporation of a hydrophilic rendering compound onto an interior surface of the o pores, the compound being a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate compound, and the compound having a solubility in water of less than about 10^"6 moles per liter. In a preferred embodiment, the body has a bubble pressure of greater than 5 pounds per square inch ("p.s.i.") 34.47 Kpa, liquid water permeability of greater than 10 x 10^"16 square meters and a loading of the hydrophilic rendering compound of greater than 15 mg. per gram of the porous carbon body.

By use of the hydrophobic resin mixed with the graphite powder and carbon fiber, the porous carbon body as described achieves long term chemical stability for 5 operation in a PEM fuel cell operating up to 1,000 - 2,000 amps per square foot

("ASF") (1.1 to 2.15 amps per square cm), without any need for time consuming and costly high temperature treatments to graphitize the body. By incorporation of the hydrophilic compound to an adequate loading onto the interior surface of the pores defined by the combined graphite, carbon fibers and resin, the body remains wettable o during long term operation of the fuel cell.

The porous carbon body may be efficiently made by first mixing together an electrically conductive graphite powder in an amount between 40% - 60% by weight of the mixture, a carbon fiber in an amount of between 20% - 40% by weight of the mixture, and a hydrophobic binder in an amount of between 10% - 30% by weight of 5 the mixture; then simultaneously compressing and heating the mixture in a mold at a pressure of between 100 - 2,500 p.s.i. (689 Kpa to 17,240 Kpa) and at a temperature of between 300 - 500°F (149°C to 260 "C) for between 3 - 20 minutes to form a body having a mean pore size of greater than 2.0 microns and an open porosity of greater than 25%; and, then rendering pores defined within the body hydrophilic by first o impregnating the pores defined within the molded porous carbon body with a solution of a salt of a metal, the oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate of which metal has a solubility in water of less than about 10^"6 moles per liter, and second heating the impregnated porous carbon body to a temperature of about between 250 - 300°F (121°C to 149°C) to hydrolyze the salt. In a preferred 5 method of manufacture, the step of heating the porous carbon body includes heating the body to a temperature high enough and heating for a period of time long enough to hydrolyze the metal salt to a metal hydroxide, and to a temperature less than a heat deflection temperature of the hydrophobic binder to enhance conductivity of the body. By mixing a hydrophobic binder with the graphite and carbon fiber in the o aforesaid proportions, compressing and heating the mixture into a molded porous carbon body having a mean pore size greater than 2.0 microns and an open porosity greater than 25% and then rendering the pores defined within the body hydrophilic as described, the porous carbon body of the present invention may be efficiently manufactured without the known costly and time consuming high temperature heating 5 undertaken to graphitize many known porous carbon bodies used in fuel cells. The resulting porous carbon body also exhibits appropriate bubble pressure, water permeability, electrical resistivity, thermal conductivity, compressive and flexural strength to efficiently serve as a water transport plate, separator plate or related component of a PEM fuel cell operating at 1,000 - 2,000 ASF (1.1 to 2.15 amps per 5 square cm) for a very long time period.

Accordingly, it is a general object of the present invention to provide a porous carbon body for a fuel cell and a method of manufacture of the body that overcomes deficiencies of prior art porous bodies for fuel cells.

It is a more specific object to provide a method of manufacturing a porous 0 carbon body for a fuel cell without a time consuming, high temperature treatment to graphitize the body.

It is yet another object to provide a porous carbon body for a fuel cell that may serve as a water transport plate, reactant fluid separator plate, reactant flow field plate within the fuel cell, electrode catalyst support layer, and/or end plate between fuel s cells.

It is still a further object to provide a porous carbon body for a fuel cell that is highly wettable by product water formed during operation of the fuel cell and or by coolant water passing through the fuel cell.

It is another specific object to provide a porous carbon body for a fuel cell that o has an extended useful life and does not degrade during operation of the fuel cell over long time periods.

These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.

5 Brief Description of the Drawings

Figure 1 is a cross-sectional, schematic representation of a fuel cell employing a porous carbon body constructed in accordance with the present invention.

Figure 2 is a graph showing degradation of conductivity of a porous carbon body as a function of a drying temperature employed in rendering pores of the body o hydrophilic.

Figure 3 is a graph showing per cent fill of a void volume of a porous carbon body as a function of loading of a hydrophilic rendering compound. Description of the Preferred Embodiments

Referring to the drawings in detail, FIG. 1 shows a schematic, cross-sectional representation of a fuel cell means for generating electrical energy from process oxidant and reducing fluid reactant streams that is generally designated by the reference numeral 10. The fuel cell 10 has a porous carbon body constructed in accordance with the present invention in the form of a first or anode water transport plate 12 and a second or cathode water transport plate 14. The anode and cathode water transport plates 12, 14 are at opposed sides of the fuel cell 10, which includes a membrane electrode assembly ("M.E.A.") 16 that consists of an electrolyte such as a proton exchange membrane ("PEM") 18, an anode catalyst 20 and a cathode catalyst 22 secured on opposed sides of the electrolyte 18.

The fuel cell 10 may also include an anode support means that is secured between and in fluid communication with the anode catalyst 20 and the anode water transport plate 12 for passing a reducing fluid or fuel stream adjacent the anode catalyst 20. The anode support means may include one or more porous layers, any one or all of which may be wetproofed, as is well known in the art, such as a porous anode substrate 24 and a porous anode diffusion layer 26. Similarly, the fuel cell may also include a cathode support means that is secured between and in fluid communication with the cathode catalyst 22 and the cathode water transport plate 14 for passing a process oxidant stream adjacent the cathode catalyst 22. The cathode support means may include one or more porous layers, any one or all of which may be wetproofed, as is well known in the art, such as a porous cathode substrate 28, and a porous cathode diffusion layer 30. As described in the aforesaid U.S. Patent 6,024,848, the anode and cathode support means may be one or more layers of carbon-carbon fibrous composites that may be wetproofed with a hydrophobic substance such as "Teflon", in a manner well-known in the art.

The anode water transport plate 12 defines a plurality of fuel flow channels 32 that are in fluid communication with each other and with a fuel inlet 34 that receives the reducing fluid so that the fuel inlet 34 and flow channels 32 cooperate to pass the reducing fluid fuel through the fuel cell 10 in fluid communication with the anode catalyst 20. Similarly, the cathode water transport plate 14 defines a plurality of oxidant flow channels 36 that are in fluid communication with each other and with an oxidant inlet 38 that receives the process oxidant so that the oxidant inlet 38 and oxidant flow channels 36 cooperate to pass the process oxidant through the fuel cell 12 in fluid communication with the cathode catalyst 22. It is pointed out that the plurality of fuel flow channels 32 are often characterized as an "anode flow field" secured adjacent the anode catalyst, and the anode flow field may include the pore volume of the anode diffusion layer 26 and anode substrate 24. Similarly, the plurality of oxidant flow channels 36 may be characterized as a "cathode flow field", 5 and may also include the pore volume of the cathode diffusion layer 30 and cathode substrate 28.

In alternative fuel cell means, the anode and cathode flow fields may be formed instead by cavities, differing channels or grooves well known in the art and defined within fuel cell components to direct the fuel and oxidant reactant streams to 0 pass adjacent the anode and cathode catalysts 20, 22. The anode water transport plate 12 also includes a plurality of anode coolant channels 40A, 40B, 40C that deliver and remove a coolant stream to and from the plate 12, and similarly, the cathode water transport plate 14 includes a plurality of cathode coolant channels 42 A, 42B, 42C that deliver and remove a coolant stream to and from the plate 14. As shown in FIG. 1, 5 the anode and cathode water transport plates 12, 14 may be structured to cooperate with adjacent water transport plates (not shown) of adjacent fuel cells in a fuel cell stack assembly (not shown), so that the anode coolant channels 40A, 40B, 40C may cooperate in mirror-image association with coolant channels in water transport plates of an adjacent fuel cell (not shown) to form a network of coolant channels for o delivering a coolant stream to the anode and cathode water transport plates 12, 14. Additionally, as is well known in the art, as pores in the anode and cathode water transport plates 12, 14 become filled with liquid from the coolant stream, such as water or an antifreeze solution, or with product water generated by the fuel cell 10 during operation, the plates 12, 14 become impermeable to gaseous movement and 5 thus form a gas barrier or seal so that the reducing fluid and process oxidant streams do not mix.

In operation of the fuel cell 10, the fuel flow channels direct the stream of hydrogen rich reducing fluid to pass through pores of the anode diffusion layer 26 and anode substrate 24 to thereby contact the anode catalyst 20 so that the hydrogen o electrochemically reacts at the anode catalyst 20 to form protons which pass through the PEM 18 to electrochemically react with oxygen at the cathode catalyst 22 to form product water. The product water must be removed from the cathode substrate diffusion layers 28, 30 and oxidant flow channels 36 at a sufficient rate to avoid flooding of the cathode catalyst 22 and thereby permit adequate oxidant to continue 5 flowing into contact with the cathode catalyst 22. Consequently, the cathode water transport plate 14 must have appropriate pore volume, pore size and wettability to permit the aforesaid movement of the product liquid water from the cathode support means through the plate 14 into the cathode coolant channels 42A, 42B, 42C, and to simultaneously support movement of the cooling fluid from the cathode coolant 5 channels 42 A, 42B, 42C through the plate 14 and into the oxidant flow channels 36 to humidify the oxidant stream. The cathode water transport plate 14 must also be chemically stable in the acidic environment of the operating PEM fuel cell 10 for a long term (exceeding for example 2,000 hours of operation for an automotive application, and as much as 40,000 hours for a stationary application); the plate 14 0 must be capable of withstanding mechanical compressive loads necessary for sealing an ordinary cell stack assembly, such as (345 Kpa to 1379 Kpa) 50 - 200 p.s.i.; and, the plate must have adequate flexural strength to be capable of withstanding varying handling requirement for manufacture and cell stack assembly of the fuel cell 10, such as approximately 1,000 p.s.i. (6895 Kpa). s To satisfy such operating requirements, the cathode water transport plate 14 and the anode water transport plate 12 are therefore porous carbon bodies constructed in accordance with the present invention. Such an improved porous carbon body formed into the cathode or anode water transport plate 14, 12 that satisfies the aforesaid and other important requirements includes an electrically conductive o graphite powder in an amount of between 40% - 60% by weight of the body; a carbon fiber in an amount of between 20% - 40% by weight of the body; a hydrophobic binder in an amount of between 10% - 30% by weight of the body; wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25% of the body; and, wherein the pores in the body are rendered partially 5 hydrophilic by incorporation of a hydrophilic rendering compound onto an interior surface of the pores, the compound being a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate compound, and the compound having a solubility in water of less than about 10^"6 moles per liter. By the phrase "interior surface of the pores" defined within the carbon body, it is meant that the "interior o surface of the pores" is a surface exposed to and in contact with fluid flow within the pores and through the body. It is stressed that the "incorporation onto the interior surface of the pores" of the hydrophilic rendering compound according to the present invention does not move the hydrophilic rendering compound beyond the interior surface into a uniform dispersion within a solid phase of the porous carbon body. In a preferred embodiment, the porous carbon body also has a bubble pressure of greater than 5 pounds per square inch ("p.s.i.") 34.5 (Kpa), liquid water permeability of greater than 10 x 10^"16 square meters, and a loading of the hydrophilic rendering compound of greater than 15 mg. per gram of the porous carbon body. By 5 the phrase "open porosity", it is meant that the pores are open to flow of fluids through a plane defined by a longest axis of the body within the pores, as opposed to sealed pores that cannot permit through flow. For example a "through plane" flow of product water through the cathode water transport plate 14 in FIG. 1 means flow in a direction from the cathode substrate layer 28 to the cathode coolant channels 42A, i o 42B, 42C, and a through-plane flow of coolant water is in a direction from the cathode coolant channels 42A, 42B, 42C to the cathode substrate layer 28. By the phrase "mean pore size", it is meant that the measurement of "greater than 2.0 microns" is measuring mean widest diameters across the pores.

One exemplary porous carbon body according to the present invention and

15 prepared by the inventors demonstrated superlative qualities, and included the following components and manufacturing steps. The following three components were dry blended for five minutes: 50% by weight of the mixture being graphite powder, such as grade "A4421" graphite powder available from the Asbury Carbons, Inc., located in Asbury, New Jersey, U.S.A.; 30% by weight of the mixture being

20 carbon fiber, such as grade "AGM-99" carbon fiber from the aforesaid Asbury Carbons; and, 20% by weight of the mixture being polyvinylidene fluoride as the hydrophobic binder, such as grade K761, available from the Atofina, Inc. located in King of Prussia, Pennsylvania, U.S.A. The mixture was then loaded into a mold and simultaneously compression molded at a temperature of about 400°F (204°C) and at a

25 pressure of about 400 p.s.i. (2758 Kpa) for about 5 minutes to a bulk density of 1.40 grams per cubic centimeter to form a body.

To render pores within the resulting body hydrophilic, a solution of 0.5 Molar ("M") tin tetrachloride pentahydrate (SnCl • 5H₂0) in water having a pH of about 0.4 was made. The aforesaid solution could also be made from a 2/1 mixture of water

3 o and isopropyl alcohol. Immediately before impregnation of the body, ammonium hydroxide was added to the solution to bring the pH up to about 1.0. Approximately

2.8 moles of hydroxide per mole of tin was added. The body was vacuum impregnated with the solution. After impregnating, the porous body was heated to a temperature high enough, and heated for a period of time long enough to hydrolyze

35 the salt to a metal hydroxide. In this example, the body was heated to about 195°F (90.56) Kpa) for a period of about 30 minutes to hydrolyze the tin salt as well as to increase a size of the salt crystals. Sufficient humidity was maintained during the heating step to prevent drying of the solution. The impregnated body was then dried at 300°F for about two hours. The dried porous carbon body was then washed in 5 water to remove ammonium chloride from its surfaces.

The resulting porous carbon body contained tin oxyhydrate at a loading of between about fifteen to about forty mg. per gram of the porous carbon body; had a bulk density of 1.3 - 1.4 grams per cubic centimeter; had an open porosity of 30%; had a mean pore size of 2.0 - 2.4 microns; had gas bubble pressure 7 p.s.i. (48.26 o Kpa); had a liquid water permeability of 35 x 10^"16 square meters; had a through-plane electrical resistivity of 0.2 ohm-cm; had a through plane thermal conductivity of 2.0 Btu Hr (2.11 Ki per hour) per square foot (929 square cm); and a flexural strength of 2,000 p.s.i. (13,790 Kpa). The chemical stability of this porous carbon body was established by immersing it in 180°F (82.2°C) water for 2,000 hours wherein the s porous carbon body experienced a weight change of 0.05%, which is effectively no detectable weight change. Additionally, no significant changes in any other physical properties were observed. The resulting porous carbon body is therefore fully capable of satisfying all requirements for the cathode and/or anode water transport plate 14, 12 in a fuel cell operating at 1,000 - 2,000 ASF (1.1 to 2.15 amps per sq. cm). o Additional tests were done to compare the plate described above to the chemical stability of prior art water transport plates made with more wettable resins. The prior art water transport plates contained epoxy, phenolic, and vinyl ester resins as the binders and were provided by known suppliers. The specific grades of the resins are proprietary to the suppliers. The prior art plates were aged for 2,000 hours 5 as described above. It was found that plates made with the epoxy resin as a binder lost 0.43% by weight; those made with the vinyl ester resin lost 0.43% by weight; and, those made with phenolic binder lost 3.4% by weight. These data show a stability benefit for the water transport plate of the present invention made with the polyvinylidene fluoride hydrophobic binder that lost only 0.05% by weight under the o same test conditions.

It is pointed out that the exemplary hydrophobic resin described above, polyvinylidene fluoride, could be substituted by any hydrophobic binder known in the art for binding carbonaceous conductive materials that has a surface energy of less than 35 dyne/cm. Exemplary hydrophobic polymers that are believed to be 5 acceptable and their respective surface energies are: polyvinylidene fluoride (PNDF) with a surface energy of 25 dyne/cm; co-polymer of ethylene and tetrafluoroethylene (ETFE) with a surface energy of 27 dyne/cm; polyvinyl fluoride (PNF) with a surface energy of 28 dyne/cm; polypropylene with a surface energy of 29 dyne/cm; polychlorotrifluoroethylene (PCTFE) with a surface energy of 31 dyne/cm; 5 polyethylene, with a surface energy of 32 dyne/cm; and any thermoset or thermoplastic polymer that has long term stability in the operating environment of a PEM fuel cell. It is also pointed out that in rendering the porous carbon body hydrophilic, the heating step and subsequent drying steps include heating the body to a temperature that is below a heat deflection temperature of the hydrophobic binder. i o For purpose herein the "heat deflection temperature of the binder" is meant to describe a temperature at which a binder material transitions from a solid phase to a plastic phase, which is a temperature below the melting temperature for the binder. For the polyvinylidene fluoride, the deflection temperature is about 240°F (115°C) at a mechanical loading of about 260 p.s.i. (1793 Kpa).

15 FIG. 2 presents a graph plotting the impact on volume conductivity of a porous carbon body with a polyvinylidene fluoride binder subject to varying "drying temperatures" listed on the horizontal axis of the graph. For purposes of comparison in the FIG. 2 graph, a porous graphite body after the mixing and simultaneous compressing and heating steps and before the rendering partially hydrophilic steps is

2 o characterized as a "formed body". The temperatures on the horizontal axis refer to maximum temperatures of the heating step to hydrolyze the metal salt, and of the subsequent drying step. As is apparent from the plots of volume conductivity at six exemplary temperatures in the FIG. 2 graph plotted at reference numerals 44A, 44B, 44C, 44D, 44E, 44F, as the heating and or drying temperatures increase above the

25 heat deflection temperature of the hydrophobic binder, the volume conductivity expressed on the vertical axis in units of "Siemens per centimeter" decreases, thus significantly diminishing performance of the porous carbon body. As the temperature exceeds the heat deflection temperature of the porous carbon body, the volume of the body expands leading to an undesirable decrease in conductivity. Therefore, a

3 o significant aspect of the present invention is the step of heating the hydrophilic compound impregnated porous carbon body to a temperature that is adequate to hydrolyze the metal salt to a metal hydroxide but that is less than a heat deflection temperature of the hydrophobic binder in the body.

Tests of the porous carbon body of the present invention have demonstrated 35 that the above described rendering partially hydrophilic steps result in the porous carbon body absorbing at ambient pressure an amount of water equivalent to about 18 to about 28 per cent of a void volume or open porosity of the porous carbon body. FIG. 3 is a graph showing a volume per cent fill of the void volume of the porous carbon body made in accordance with the present invention as a function of tin oxide loading. As is apparent from the plots of per cent of void volume of a porous carbon body filled at three exemplary tin oxide loading rates as plotted at reference numerals 46 A, 46B, and 46C, a zero loading at 44A provides for zero fill of the void volume; a 28 milligram per gram loading of tin results in about 18 per cent fill of void volume; and, about 50 milligrams per gram loading results in about 28 per cent fill of void volume.

As shown in FIG. 3, the above described treatment results in only about 18 to about 28 per cent of the void volume being filled with water. The treated carbon bodies or plates, however, had acceptable bubble pressure and water permeability characteristics as described above, which was unexpected with the 18 per cent to 28 per cent of the void volume being filled with water. The fact that the per cent fill of the void volume of the treated plates with water is in the range of 18 to 28 per cent establishes that the body is only partially hydrophilic. A perfectly hydrophilic body would have nearly 100 per cent of its pore volume hydrophilic. It was unexpected by the inventors of this invention that a such a treated carbon body with the aforesaid relatively low level of hydrophilicity (about 18 per cent to about 28 per cent) would result in acceptable performance characteristics as a water transport plate within a fuel cell. It is stressed that water absorption of carbon body plates made as described above in accordance with the present invention, but without the rendering partially hydrophilic step, is essentially 0 per cent of the void volume of the plates. The present inventors performed a comparative experiment in an attempt to demonstrate water absorption of a carbon body made in accordance with a disclosure of an electrically conductive separator plate for a fuel cell as described in the aforesaid U.S. Patent No. 5,942,347 to Koncar et al. Koncar et al. disperses wettability enhancing materials with graphite powder, carbon fiber and a resin prior to molding the separator plate. (See Koncar et al., at Col. 5, lines 1 - 67.) In the comparative experiment, a graphite powder grade A-99 from the aforesaid Asbury Carbons, Inc. was pre-treated with tin oxide to a loading of 25 - 40 mg/gm by way of the tin oxide deposition technique previously described to enhance its wettability. The following three components were dry blended for five minutes: 45 per cent by weight of A-99 graphite powder treated with the tin oxide; 45 per cent by weight AGM-99 carbon fiber; and 10 per cent by weight polyvinylidene fluoride. This mixture was molded as previously described. The water absorption rate of the comparative experiment separator plate was measured to be essentially 0 per cent of the void volume of the plate. The comparative experiment further demonstrates the 5 substantial benefit of rendering the interior surfaces of the pores in the carbon body partially hydrophilic after the mixture is molded into the carbon body or plate in accordance with the present invention.

It is pointed out that the hydrophilic rendering compound may be oxides, hydroxides, oxyhydroxides, oxyhydroxide hydrates, or oxide hydrates of tin, titanium, 0 aluminum, zirconium, gallium, indium, niobium, tantalum, ruthenium, zinc, and/or hafnium. The general formula for the appropriate compounds is M_xO_y(OH)_w»(H₂0), wherein the H₂0 component is optional, i.e. is only included in the hydrate compounds which may be used in performance of this invention, and wherein the x, y, z, are generally small integers between zero and five, and preferably two or three, and s M is the metal. Preferred hydrophilic rendering compounds have been determined to be tin oxyhydroxide and tin hydroxide. A preferred loading range for the compound is about 25 to about 50 milligrams per gram of the porous carbon body.

It is pointed out that a preferred embodiment of this invention includes a porous carbon body having a conductive graphite powder between 40% - 60% by o weight of the body; a carbon fiber in an amount of between 20% - 40% by weight of the body; a hydrophobic binder in an amount of between 10% - 30% by weight of the body; wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25% of the body; and, wherein the pores in the body are rendered partially hydrophilic by incorporation onto the interior surface of the pores 5 of a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate compound, said compound having a solubility in water of less than about 10^"6 moles per liter. As described above, the favorable porosity and mean pore size may be produced by the simultaneous compression and heating of the graphite powder, carbon fiber and hydrophobic resin. However, it is also possible to achieve the same o favorable porosity and mean pore size by utilizing alternative procedures. For example, conductive graphite particles may be mixed with the hydrophobic binder, and with a foaming agent or a leachable solid known in the art. The mixture is then compression molded and heated to form a desired shape for the body and so that the foaming agent produces the pores. The leachable solid is subsequently leached out so 5 that the resulting porosity is an "open porosity" permitting flow through the body, as defined above. Such a formed carbon body having a mean pore size of greater than 2.0 microns and an open porosity of greater than 25% could then be rendered partially hydrophilic by incorporation onto the interior surface of the pores of the body of the aforesaid hydrophilic rendering compound, being a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate compound, said compound having a solubility in water of less than about 10^"6 moles per liter.

In another preferred embodiment, increased electrical conductivity of a porous carbon body made in accordance with the present invention has been achieved by including a high conductivity carbon black with the graphite powder, carbon fiber, and hydrophobic binder. Exemplary high conductivity carbon blacks include "VULCAN XC-72", and "BLACK PEARL 2000", manufactured by Cabot Corporation of Boston, Massachusetts, U.S.A., or "KETJEN BLACK", made by the Ketjen Black International Company of Tokyo, Japan. Experiments were performed to identify preferred ranges of a high conductivity carbon black mixed with the graphite powder, carbon fiber and hydrophobic binder, the results of which are included in TABLE 1 herein below.

TABLE 1

As can be seen, utilization of 20 per cent, or an optimal range of 10 - 30 per cent high conductivity carbon black results in substantially enhanced conductivity, and also provides acceptable mean pore size, open porosity, flexural strength, and other requirements for a porous carbon body appropriate for utilization within a PEM 5 fuel cell.

In the aforesaid preferred embodiments, by use of the hydrophobic resin mixed with the graphite powder and carbon fibers, the porous carbon body as described achieves long term chemical stability for operation in a PEM fuel cell operating up to 1,000 - 2,000 amps per square foot ("ASF") (1.1 to 2.15 amps per o square cm), without any need for time consuming and costly high temperature treatments to graphitize the body. By incorporation of the hydrophilic compound to an adequate loading onto the interior surface of the pores defined by the combined graphite, carbon fibers and resin, the body remains wettable during long term operation of the fuel cell. The porous carbon body resulting from the described s efficient manufacturing process also exhibits appropriate bubble pressure, water permeability, electrical conductivity, thermal conductivity, compressive and flexural strength to efficiently serve as a water transport plate, separator plate or related component of a PEM fuel cell operating at 1,000 - 2,000 ASF (1.1 to 2.15 amps per square cm) for a very long duration. o While the present invention has been described and illustrated with respect to particular embodiments and methods of manufacture of a porous carbon body for use in a fuel cell 10, it is to be understood that the present invention is not to be limited to the described and illustrated embodiment. For example, although the porous carbon body of the present invention has been primarily described in the context of a "PEM" 5 fuel cell, the body is applicable to other fuel cells utilizing other solid polymer or aqueous electrolytes. Further, although FIG. 1 shows schematically a single fuel cell including two porous carbon bodies of the present invention in the form of the anode and cathode water transport plate 12, 14 components, the invention includes application of the porous carbon body as differing fuel cell components such as o separator plates, support plates, end plates, etc., and the invention also contemplates usage of the porous carbon body in a plurality of fuel cells cooperatively secured in a well known fuel cell stack. Accordingly, reference should be made primarily to the following claims rather than the foregoing description to determine the scope of the invention.

Claims

1. A porous carbon body for use in a fuel cell, the body comprising: a. an electrically conductive graphite powder in an amount of between 40% - 60% by weight of the body; b. a carbon fiber in an amount of between 20% - 40% of the body;

5 c. a hydrophobic binder in an amount of between 10% - 30% by weight of the body; d. wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25% of the body; and, e. wherein the pores in the body are rendered partially hydrophilic by l o incorporation of a hydrophilic rendering compound onto an interior surface of the pores, the compound being a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate compound, and the compound having a solubility in water of less than about 10^"6 moles per liter.

2. The porous carbon body of Claim 1 , wherein the hydrophobic binder has a surface energy of less than 35 dynes/cm.

3. The porous carbon body of Claim 1 , wherein the hydrophobic binder is polyvinylidene fluoride.

4. The porous carbon body of Claim 1 , wherein the body has a bubble pressure of greater than 5 pounds per square inch (34.47 Kpa).

5. The porous carbon body of Claim 1 , wherein the hydrophilic rendering compound is an oxide, hydroxide oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate of a metal selected from the group consisting of tin, titanium, aluminum, zirconium, gallium, indium, niobium, tantalum, ruthenium, zinc, and hafnium.

6. The porous carbon body of Claim 5, wherein the metal is tin.

7. The porous carbon body of Claim 1 , wherein the hydrophilic rendering compound is loaded onto the interior surface of the pores at a loading greater than 15 milligrams of compound per gram of the porous carbon body.

8. The porous carbon body of Claim 1 , wherein the pores of the body are sufficiently hydrophilic to absorb an amount of water equivalent to about 18 to about 28 per cent of a void volume of the body.

9. A porous carbon body for use in a fuel cell comprising: a. an electrically conductive graphite powder in an amount of between 60% - 80% by weight of the body; b. a hydrophobic binder in an amount of between 20% - 40% by weight of the body; c. wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25% of the body; and, d. wherein the pores in the body are rendered partially hydrophilic by incorporation of a hydrophilic rendering compound onto an interior surface of the pores, the compound being a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate compound, the compound having a solubility in water of less than about 10^"6 moles per liter, and the compound being loaded onto the interior surface of the pores at a loading greater than 15 milligrams of compound per gram of the porous carbon body.

10. The porous carbon body of Claim 9, wherein the hydrophobic binder has a surface energy of less than 35 dynes/cm.

11. The porous carbon body of Claim 10, wherein the hydrophobic binder is polyvinylidene fluoride.

12. The porous carbon body of Claim 11, wherein the body has a bubble pressure of greater than 5 pounds per square inch. (34.47 Kpa).

13. The porous carbon body of Claim 12, wherein the hydrophilic rendering compound is an oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate of a metal selected from the group consisting of tin, titanium, aluminum, zirconium, gallium, indium, niobium, tantalum, ruthenium, zinc, and hafnium.

14. The porous carbon body of Claim 13, wherein the metal is tin.

15. The porous carbon body of Claim 14, wherein the pores of the body are sufficiently hydrophilic to absorb an amount of water equivalent to about 18 to about 28 per cent of a void volume of the body.

16. A porous carbon body for use in a fuel cell, the body comprising:

> a. an electrically conductive graphite powder in an amount of between 30% - 50% by weight of the body; b. a carbon fiber in an amount of between 15% - 35% of the body; c. a high conductivity carbon black in an amount of between 10% - 30% by weight of the body; d. a hydrophobic binder in an amount of between 10% - 20% by weight of the body; e. wherein the body has a mean pore size of greater than 2.0 microns, and an open porosity of greater than 25% of the body; and, f. wherein the pores in the body are rendered partially hydrophilic by incorporation of a hydrophilic rendering compound onto an interior surface of the pores, the compound being a metal oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate compound, and the compound having a solubility in water of less than about 10^"6 moles per liter.

17. The porous carbon body of Claim 16, wherein the hydrophobic binder has a surface energy of less than 35 dynes/cm.

18. The porous carbon body of Claim 16, wherein the hydrophobic binder is polyvinylidene fluoride.

19. The porous carbon body of Claim 16, wherein the body has a bubble pressure of greater than 5 pounds per square inch (34.47 Kpa).

20. The porous carbon body of Claim 16, wherein the hydrophilic rendering compound is an oxide, hydroxide oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate of a metal selected from the group consisting of tin, titanium, aluminum, zirconium, gallium, indium, niobium, tantalum, ruthenium, zinc, and hafnium.

21. The porous carbon body of Claim 20, wherein the metal is tin.

22. The porous carbon body of Claim 16, wherein the hydrophilic rendering compound is loaded onto the interior surface of the pores at a loading greater than 15 milligrams of compound per gram of the porous carbon body.

23. The porous carbon body of Claim 16, wherein the pores of the body are sufficiently hydrophilic to absorb an amount of water equivalent to about 18 to about 28 per cent of a void volume of the body.

24. A method of forming a porous carbon body for use in a fuel cell, the method comprising the steps of: a. mixing together an electrically conductive graphite powder in an amount of between 40% - 80% by weight of the mixture, and a hydrophobic binder in an amount of between 10% - 40% by weight of the mixture; b. then simultaneously compressing and heating the mixture in a mold at a pressure of between 100 - 2,500 pounds per square inch (17,240 Kpa) and at a temperature of between 300 - 500 degrees Fahrenheit (149°C to 260°C) to form a body having a mean pore size of greater than 2.0 microns and an open porosity of greater than 25%; and, c. then rendering pores defined within the body partially hydrophilic by impregnating an interior surface of the pores defined within the molded body with a solution of a salt of a metal, the oxide, hydroxide, oxyhydroxide, oxyhydroxide hydrate, or oxide hydrate of which has a solubility of less than about 10^"6 moles per liter, and then heating the impregnated porous carbon body to a temperature of between about 250 - 300 degrees Fahrenheit (149°C to 260°C) for a period of time adequate to hydrolyze the metal salt to a metal hydroxide.

25. The method of Claim 24 wherein said step of heating the impregnated porous carbon body further comprises the step of heating the body to a temperature less than a heat deflection temperature of the hydrophobic binder.

26. The method of Claim 24, wherein the step of impregnating is performed under a vacuum.

27. The method of Claim 24, wherein the metal is a metal selected from the group consisting of tin, titanium, aluminum, zirconium, gallium, indium, niobium, tantalum, ruthenium, zinc, and hafnium.

28. The method of Claim 27, wherein the metal is tin, and wherein the step of heating the impregnated porous carbon about 250 - 300 degrees Fahrenheit (149°C to 260°C) is sustained for greater than 30 minutes.

29. The method of Claim 24, wherein the mixing together step further comprises mixing with the electrically conductive graphite powder and hydrophobic binder a carbon fiber in an amount of between 20% - 40% by weight of the mixture.

30. The method of Claim 24, wherein the mixing together step further comprises mixing with the electrically conductive graphite powder and hydrophobic binder a high conductivity carbon black in an amount of between 10% - 30% by weight of the mixture.